1. Field of the Invention
This invention relates to a birefringent plate using an obliquely evaporated film and a manufacturing method for the same.
2. Discussion of the Background
Heretoforem, there has been known a birefringent plate having an obliquely evaporated film made of a dielectric material formed on a substrate. This birefringent plate has been manufactured by a vacuum film forming method such as a vacuum evaporation method, where columnar structure of the dielectric material are obliquely grown on the substrate surface disposed obliquely to the direction of the dielectric material (evaporation material) flying from an evaporation source. Thus obtained obliquely evaporated film has been useful for optical function devices such as a quarter wavelength (.lambda.) plate and the like because the film has birefringence to an incident light. However, there has been a defect that the applications of the optical function devices using birefringence of the obliquely evaporated fiom are restricted by their visual angle dependence. That is, as shown in FIG. 4, an obliquely evaporated film 1 made of the dielectric material has columnar structure growing obliquely on the surface of a substrate 2. Therefore, the obliquely evaporated film 1 exhibits relatively large birefringence to the incident light from the direction (a). However, the birefringence becomes small when the direction of the incident light is changed from (a) to (b) or (c). Whether the birefringence becomes large or small depends on the direction of the incident light (visual angle). Further, it has been difficult to obtain uniform thickness of the obliquely evaporated film by the method in which the substrate is disposed obliquely to the direction of the dielectric material flying from the evaporation source. That is, as shown in FIG. 2, the obliquely evaporated film has been formed on the substrate 2 by holding the substrate 2 at an angle .theta. to and at some distance away from a crucible 17 (evaporation source). The crucible 17 is placed horizontally in the lower part of a vacuum chamber 9. Accordingly, the film thickness has tended to be large on the portion of the substrate surface much closer to the crucible 17 due to the distance difference between the portions of the substrate surface and the crucible 17. Also, to keep some distance between the substrate 2 and the crucible 17 has required a large-sized device which takes much time to evacuate air from the chamber 9 with low deposition rate.